The present invention relates generally to nondestructive evaluation of engineering materials, and more particularly to using ultrasonic techniques for detecting ply wrinkling in a laminated composite.
Typically, a laminated composite consists of several layers of plies laminated together. A common problem that often arises during the manufacture of a thick composite by compression molding is that plies may wrinkle. Ply wrinkling occurs when the compressive force on the composite exceeds the binder strength, resulting in a ply wrinkle. Ply wrinkling in a laminated composite is a major problem because of its potential to compromise mechanical properties of an intended design. In particular, a laminated composite with ply wrinkling is more susceptible to tensile and bending forces, making it impractical for the composite to perform its intended function, resulting in a lesser quality product.
Therefore, there is a need for a procedure that can accurately detect and measure wrinkles in a laminated composite. In particular, it is important to measure the severity and depth of wrinkles because wrinkles near the surface of the composite are much more susceptible to bending forces. One procedure for detecting wrinkles in components has been disclosed in Pereira et al., Sensing Strategy to Detect Wrinkles in Components, IEEE TRANSACTION ON INSTRUMENTATION AND MEASUREMENT, Vol. 43, No. 3, June 1994, pp. 442-448. Pereira et al. uses two displacement sensors (i.e., either laser or fiber optic) to detect the maximum amplitude of a wrinkle present in a component. The effectiveness of the procedure disclosed in Pereira et al. is limited to wrinkles that are visible on the surface and not wrinkles within the material. Thus, Pereira et al. is unable to fully characterize the severity and depth of any subsurface wrinkles in a composite, making it hard to ascertain the extent of how much a design has been compromised. In another procedure, wrinkle detection and measurement is attained by using an ultrasound transducer operating in a transmission or normal incidence pulse echo mode and imaging the ultrasound attenuation caused by the wrinkle. In this procedure, only the shadow of the wrinkle is detected and not the wrinkle. Consequently, the severity and depth of the wrinkle cannot be fully characterized.